An echelle spectrograph is a spectrograph which uses an echelle grating to diffract light at high resolutions and high diffraction orders. As with other blazed diffraction gratings, the echelle grating consists of a number of grooves, the width of the grooves being close to the wavelength of the diffracted radiation. However, echelle gratings are specifically characterized by the large spacing between the grooves and, therefore, comprises a lower groove density.
Light incident upon any blazed grating is split into several different diffraction orders. Each order will be comprised of a different but overlapping wavelength range. The dispersion associated with each order will also be different. The overlapping orders make it difficult to associate the correct order numbers with their wavelength ranges. This ambiguity complicates the spectrum and makes it more difficult to determine the correct wavelength emission from the source.
Although this overlap is generally an unwanted side effect, echelle gratings specifically use this effect to enhance the performance of the spectrograph. A second cross-dispersing element is used to spatially separate the orders. The individual orders, each with a separate (and sometimes overlapping) wavelength range and resolution, can then be analyzed without ambiguity.
Typical echelle spectrographs have a relatively high effective fvalue, generally f/7 or greater, limiting the total light which reaches the image plane and thereby decreasing the resulting image quality. Further, the high effective fvalue of typical echelle spectrographs prevent their use in certain applications such as Raman spectroscopy where the detection of weak emissions requires the use of a spectrograph with a very low fvalue. Clearly, it is desirable to design an echelle spectrograph with a low fvalue.